Terahertz waves typically are electromagnetic waves having components of any frequency bands within a range of 0.03 THz to 30 THz. Such frequency bands include many characteristic absorption bands derived from structures or states of various substances including biomolecules. Using such characteristics, inspection techniques of non-destructive analysis or identification of substances have been developed. Also, application of terahertz waves to safe imaging techniques replacing X-rays or high speed communication techniques is expected. Further, it is noted that reflection terahertz waves from a refractive index interface in an object to be measured are applied to a tomography apparatus configured to visualize an inside of the object to be measured. Such an apparatus is expected to visualize an inner structure at a depth of several hundreds of μm to several tens of mm using a characteristic of transmission of terahertz waves. In many cases, terahertz waves in the form of subpicosecond pulses are used for such use. Usually, it is difficult to obtain such pulses in an actual time. Thus, a THz-TDS apparatus (terahertz time-domain spectroscopy apparatus) performs sampling measurement using ultrashort pulsed lights (also herein referred to as excitation lights) having a femtosecond pulse width. Sampling of the terahertz waves is achieved by adjusting an optical path length difference between excitation lights reaching a generation unit configured to generate the terahertz waves and a detection unit configured to detect the terahertz waves. For example, the optical path length difference is adjusted according to an amount of fold of the excitation lights by inserting a stage (also herein referred to as a delay optical unit) having a folded optical system into propagation paths of the excitation lights. In many cases, a photoconductive element having an antenna electrode pattern with a minute gap provided on a semiconductor thin film is used as the generation unit or the detection unit.
In an example of the present invention, physical properties of an object to be measured are obtained using the principle of a THz-TDS apparatus. The physical properties of the object to be measured mainly include a refractive index and a shape (thickness) of the object to be measured. These physical properties are often calculated using terahertz wave pulses, from a time difference between reflection pulses from a refractive index interface of the object to be measured (see PTL 1). An interval between the reflection pulses corresponds to an optical length of propagation of terahertz waves. The optical length of the terahertz waves is expressed in the form of n(ave)×t by multiplying a thickness t of the object to be measured by an average refractive index n(ave). The average refractive index refers to a typical refractive index of the object to be measured. For example, the average refractive index refers to an average value of refractive index dispersion in a frequency band used. Otherwise, the average refractive index refers a refractive index at frequency (wavelength) having highest intensity in a frequency spectrum of the object to be measured. It is difficult to calculate the thickness t and the average refractive index n(ave) of the object to be measured from a measurement result of the terahertz waves because there is only one measurement result for two unknowns. To solve this problem, PTL 1 calculates a refractive index of an object to be measured by another unit. As such, to separate the thickness t and the average refractive index n(ave) from the measurement result, either of them needs to be obtained using another measurement unit. For this purpose, properties of the object to be measured are desirably specified to some degree before measurement. However, such an act may limit an application range as a measurement apparatus.
Meanwhile, for an OCT apparatus (Optical Coherence Tomography apparatus), some techniques for separating such information have been developed as described below. A confocal optical system is constructed, and a thickness t and an average refractive index n(ave) are simultaneously calculated from, an amount of movement of a focus in a collecting position of the confocal optical system in a depth direction of an object to be measured, and an amount of change of an optical path length difference of an interference system required for obtaining a maximum interference signal for each focus. Specifically, the thickness t and the average refractive index n(ave) of the object to be measured are calculated from a ratio of the amount of change of the optical path length difference to the amount of movement of the focus (see PTL 2).